Extraction of whole spherules of Coccidioides immitis grown in vitro was carried out to obtain a cell free antigen that could provide protection in mice against lethal challenge with viable C. immitis. Whole spherules were disrupted in a Braun homogenizer and the walls extracted with phosphate buffered saline with or without prior treatment with chitinase. An immunogenic acellular antigen representing only 3 to 5% of the original whole spherules was obtained that only in the presence of alum adjuvant provided protection against death at some doses of intranasal (IN) challenge. Chitinase treatment appeared to diminish the immunizing activity of the extract. A dose response study indicated that 3 doses of 0.55 to 1.1 mg provided substantial protection against IN challenge with 1000 arthrospores. The sensitizing effect (to coccidioidin and spherulin) of small doses of whole killed spherules appeared diminished by the inclusion of 1 or 2 doses (0.1 mg) of the extract from spherule walls.